@Sealab2015so could you cast a shield spell from a scroll as a reaction? Yes.
— Jeremy Crawford (@JeremyECrawford) October 24, 2016
@Sealab2015so could you cast a shield spell from a scroll as a reaction? Yes.
— Jeremy Crawford (@JeremyECrawford) October 24, 2016
Yes, *IF* you have the scroll in hand already. Otherwise you have to retrieve it from wherever it’s stored.
It doesn’t say anywhere you need a separate action to draw it or have it in you hand. Regardless of the casting time current RAW on scrolls seems to be you can retrieve them as part of the action of the spell. Technically you could even cast from a scroll hanging off the wall in front of you. it says READ the scroll, nothing else is required. It’s not even clear if you need to be looking at it when you cast it or if you can read it from memory while it’s on your person. Needs More clarification. But really just let them use it as a reaction, because it’s really easy to get around the in hand requirement. Nail it to you shield, sew it to your sleeve. It’s a reaction, full stop, nothing else needed. That’s how JC is with rulings. They do exactly what they say.
By RAW there is no extra requirement to retrieve a scroll when you use it. The wording doesn’t say anything about that at all. Given JC’s ruling it seems that the action of using a scroll includes retrieving it. Scrolls don’t even require you to hold them to work, just READ them, by RAW you can cast from an open scroll lying on a table in front of you perfectly fine. You can even cast from a massive distance if you can read it. That may not make sense but it is the RAW. So unless you really want to make players staple scrolls to the backs of their shields or sew/glue them on their clothes (which is actually a cool idea now that I think about it), just let them use scrolls as part of the casting action. Really if they can’t be used as stated then they are basically useless, and if the DM requires you to have them in your hand but won’t let you bypass that by putting them on the back of your shield or something then it’s obviously just them ignoring RAW because they don’t think reaction scrolls should exist. Just let them work, otherwise why allow them at all?